Somehow, he needs to have it, besides a leech weapon. Imagine fighting dragon with a group, he'll go through SP in a single fight and then say "brb guys i gotta kill 20 mobs to refill my sp" every time. Its no fun and thats basically his best damage potential gone, knights hurt PLENTY enough to DPS instead

I get that its a disadvantage of theirs but it can stay like that. Make a skill called Metabolism, having a whole different skill equipped/trained just to get SP back is a penalty. Rapidly drain food reserves to restore SP. This also means carrying a lot more food on a barbarian. If there are ANY balance issues with it, i'd rather Adrenalin or Lifetap be nerfed, because every spell and ability sucks SP so much i had to pump meditation as a tertiary. Restoration potions arnt a great option either, very paltry SP, cooldown, high cost. 

 

I checked my list of Skills and i see that adding this skill gives it heavy competition and this is before dual wielding is added. This would replace something vital ike Poison, poison resistance, druidery or Hide for late game, so they are not short on skills to wear.

 

You could say its 'just like concentration for barbs' but we still lose the MP regen and its not a passive increase. The class is not very popular compared to knight anyways. Rogue/Ranger seem to be too but someday i might take a look at them !